1. Field of the Invention
The present invention is directed to ultra wide bandwidth communication systems and, more particularly, to a receiver included in ultra wide-band communications systems.
2. Description of the Related Art
An ultra wide bandwidth spread-spectrum communications system is disclosed in U.S. Utility patent application Ser. No. 09/209,460 filed Dec. 11, 1998. In the ultra wide bandwidth spread-spectrum communication system disclosed in the foregoing application, data is transmitted between a transmitter and a receiver through multiple pathways. Transmission of data from a transmitter to a receiver through multiple pathways is referred to as multi-path communications.
The transmitted data is encoded by the transmitter using sequences of N wavelets of particular shapes and positions. Each of the sequences of N wavelets is referred to as an N-length code and represents one bit of digital data, either logical “1” or logical “0”. Whether a particular sequence of wavelets represents a logical “1” or a logical “0” is selected arbitrarily according to the source data at the transmitter. For example, a logical “1” would be represented by the sequence of wavelets shown in FIG. 1A, while a logical “0” would be represented by a the same sequence of wavelets inverted as shown in FIG. 1B.
As shown in FIGS. 1A and 1B, the period of each wavelet of each waveform RF is 500 picoseconds (ps), with some arbitrary delay occurring between each wavelet referred to as a chip period. That is, a chip period refers to time from one wavelet to a corresponding location on the next wavelet, such as center-to-center. The N-length code is transmitted between t=t0 and t=T.
FIG. 2 shows a receiver correlator structure 10 which receives data organized into N-length codes. The data, carried on signal RF, impinges on an antenna 12, and is transmitted to coupled mixer 14. Mixer 14 also receives a local oscillator signal, LO, generated by a wavelet generator (not shown in FIG. 2).
Signal LO is a pulse stream divided into N-length codes, matched in time to the N-length codes of signal RF. Each N-length code of signal LO represents the same logical value as the prior four-length code of signal LO. That is, the signal LO provides the same value, either logical “0” or logical “1” code as sent by the transmitting source device, to mixer 14. Mixer 14 mixes signal RR with signal LO to produce signal IF=RF*LO.
An example of a waveform for signal LO, having a four-length code corresponding to logical “0” code, is shown in FIG. 1B traveling between time t=t0 and t=T.
FIG. 3 shows the simplest code in which every chip is the same.
FIGS. 4A and 4B show example waveforms corresponding to signal IF at the output of mixer 14. The resultant IF waveform shown in FIG. 4A is produced when signal RF, having a value of logical “1” code shown in FIG. 1A, is mixed by mixer 14 with signal LO also having a value of logical “1” code. The resultant IF waveform shown in FIG. 4B is produced when signal REF having a value of logical “0” code shown in FIG. 1B is mixed by mixer 14 with signal LO having a value of logical “1” code.
Signal IF is then integrated by integrator 16 and, subsequently, converted to a digital signal D by analog-to-digital (A/D) converter 18. Digital signal processor (DSP) 20 then removes noise from the resultant integrated digital signal by implementing algorithms known to one of ordinary skill in the art.
Mixer 14, integrator 16, analog-to-digital convertor 18 and digital signal processor 20 are conventional components.
Mixer 14 and integrator 16 comprise, ideally, a mathematically matched filter for signal RF. Moreover, integrator 16 is aligned correctly with each bit of data and as would be apparent to one of ordinary skill in the art, will either ramp up or ramp down, depending upon the value corresponding to a particular bit integrated by integrator 16.
Since imperfect isolation occurs between signal LO and signal RF at mixer 14, resulting in leakage current 22 between signal LO and signal RF, a bias in signal IF is included at the output of mixer 14.
FIGS. 5A-5C show graphs of relative values of signals IF, I, and D shown in FIG. 2. As shown in FIGS. 5A-5C, if signal IF is as shown in FIG. 4A, then the waveform of signal I would appear as shown in FIG. 5B, and the waveform of signal D would appear as shown in FIG. 5C. The waveform of signal I shown in FIG. 5B includes an upward slope from 0V to 1V, from 1V to 2V, etc. due to both DC and AC components present in signal I, which were introduced by signal LO into mixer 14 and integrated by integrator 16.
At time t=t0, and at each subsequent sampling point through t=T executed by integrator 16, the value of signal I increases by 1, but that increase is a sloped increase due to DC components included in I, until a value of “4” is reached at time t=T. Between t=t0 and t=T, the value of Di is 0. At time t=T, A/D 18 samples I and increases the value of D to D=4. That is, Di+1=4. Thereafter, the integrator 16 is reset to ground causing D to be reset to 0. That is, Di+2=0.
Referring again to FIG. 2, when signal LO is transmitted to mixer 14, leakage current 22 results in a DC offset (or bias) being provided to signal IF at the output of mixer 14, and the resultant DC offset is integrated by integrator 16. Moreover, the value of signal LO is relatively large being typically in the range of volts) compared to the value of signal RF (being typically in the range of microvolts), making it difficult for receiver correlator structure 10 to distinguish between signal LO and signal RF after signals LO and RF are mixed by mixer 14.
More particularly, signal LO interferes with signal RF by coupling to RF, by radiating into the air and thus impinging upon antenna 12, and by radiating to and bouncing back from antenna 12.
Moreover, signal LO when provided to mixer 14 causes the above-mentioned bias to change randomly over time, resulting in mixer 14 having an output quantity of noise proportional to 1/f, where f is a frequency.
A problem with the ultra wide bandwidth receivers of the related art is that the mixer may transmit a leakage current from the local oscillator signal (which is typically in the range of microvolts) to the input of the mixer which receives the pulses, resulting in a DC offset (or bias) from the leakage current being provided at the output of the mixer. This bias then propagates as noise through the rest of the receiver, and interferes with the decoding of the information carried on the pulses.
Another problem with ultra wide bandwidth receivers of the related art is that the analog-to-digital converter may not sample the waveform output by the bandpass receiver at a sample point which corresponds to the maximum height of the waveform.